Nowadays, since, for example, automobile exhaust gas regulations are requested in response to increasing concern about global environment problems, weight reduction of automobile bodies is a very important issue to be solved. Decreasing the thickness (thickness reduction) of a steel sheet by increasing the strength of a steel sheet is an effective method for reducing the weight of automobile bodies. Nowadays, as a result of a significant increase in the strength of a steel sheet, there is an active trend toward using a thin steel sheet having a tensile strength TS of 780 MPa or more and a thickness of less than 2.0 mm. However, since there is a problem of a decrease in the rigidity of automobile bodies due to thickness reduction, it is necessary to increase the rigidity of the structural parts of automobiles. Since the rigidity of structural parts having the same cross-section shape varies depending on the thickness and Young's modulus of a steel sheet, increasing the Young's modulus of a steel sheet is effective for achieving both sufficient weight reduction and satisfactory rigidity for structural parts.
It is known that Young's modulus varies greatly depending on the texture of a steel sheet and that, in the case of iron, which has a body-centered cubic lattice, Young's modulus is high in the <111> orientation, in which atom density is high, and conversely low in the <100> orientation, in which atom density is low. It is known that the Young's modulus of ordinary iron having no grain orientation anisotropy is about 206 GPa. In addition, by increasing atom density in a specific direction as a result of providing grain orientation anisotropy to iron, it is possible to increase Young's modulus in this direction. However, with regard to the rigidity of automobile bodies, since loads are applied from various directions, a steel sheet is required to have a high Young's modulus in all directions instead of in a specific direction.
In order to meet such a requirement, for example, Patent Literature 1 proposes a method for manufacturing a high strength thin steel sheet excellent in terms of rigidity, the method including hot-rolling a slab having a chemical composition containing, by massa, C: 0.02% to 0.15%, Si: 0.3% or less, Mn: 1.0% to 3.5%, P: 0.05% or less, S: 0.01% or less, Al: 1.0% or less, N: 0.01% or less, Ti: 0.1% to 1.0%, and the balance being Fe and inevitable impurities, cold-rolling the hot-rolled steel sheet with a rolling reduction of 20% to 85%, and then performing recrystallization annealing in order to obtain a steel sheet having a microstructure of a ferrite single phase, a TS of 590 MPa or more, a Young's modulus of 230 GPa or more in a direction at an angle of 90° to the rolling direction, and an average Young's modulus in directions at angles of 0°, 45°, and 90° to the rolling direction of 215 GPa or more.
Patent Literature 2 proposes a method for manufacturing a high-rigidity high strength steel sheet excellent in terms of workability, the method including hot-rolling a slab having a chemical composition containing, by mass %, C: 0.02% to 0.15%, Si: 1.5% or less, Mn: 1.5% to 4.0%, P: 0.05% or less, S: 0.01% or less, Al: 1.5% or less, N: 0.01% or less, Nb: 0.02% to 0.40%, and the balance being Fe and inevitable impurities, cold-rolling the hot-rolled steel sheet with a rolling reduction of 50% or more, and then performing recrystallization annealing in order to obtain a steel sheet having a mixed microstructure of ferrite and martensite, a TS of 590 MPa or more and a Young's modulus of 225 GPa or more in a direction at a right angle to the rolling direction.